If you view the finished image outside of GIMP, e.g. on IrfanView or some other picture viewer, does it still look the same?
is it Gimp, Ubuntu OS, the graphics cards, the monitor settings?
-Gimp: I have never heard of that problem in Gimp, you might try to reinstall it and see if it's the same.
-Ubuntu: Well .. no. I don't think Ubuntu has got anything to do with this. If you want to be a 100% sure, send the files to a Windows/Mac computer and see whether it comes out the same way.
-Graphics Card: As above, I don't think the graphics card would effect a data. It could show you something in a wrong way, but if you print the image, the GPU has no further influence and the picture therefor should be colored correctly. As I said above, try it on another Computer to get a 100% proof.
-Monitor settings: Same here as on the GPU.
Well, I've never had this problem, and I haven't heard about it either. Are you sure you didn't do any mistakes while making the XCF in GIMP? For example some layer with a transparent-to-white gradient on low opacity that gives the whole image this look?
If you haven't got another computer and if you trust us well enough, you could upload the XCF file here and we'll have a look at it to see whether there's a mistake inside.
P.S: Any grammar mistakes are intended.